Today the House of Lords debates the proposal from the Home Office to reclassify cannabis from class C to class B. In recommending this change to parliament, the government has rejected the explicit advice of its appointed experts, the Advisory Council on the Misuse of Drugs, for the first time in nearly 30 years. In its last report, produced at the request of the home secretary, the ACMD clearly recommended - for the third time in the last six years - that cannabis remains a class C drug, and did so after examining all the available and latest evidence on short- and long-term health risks, as well as social harms, public attitudes and policing priorities.

After setting out its conclusions on the health risks of cannabis and concerns regarding greater potency, the report made clear recommendations for improved drugs education and greater efforts to tackle drug dealing. However, it concluded that the evidence was against greater criminalisation of possession. The impact of parliament agreeing to the government's policy could be very damaging. Cannabis use has fallen in recent years, especially following its downgrading to class C in 2004, and it is obviously unwise to risk reversing that trend. The classification system must be credible - reclassification would send out an ambiguous message about the dangers of current class B drugs.

Even more importantly, the move would be a sad departure from the welcome trend - established after the Phillips report into the BSE disaster - of public policy following expert scientific advice unless there is new evidence. Baroness Meacher has tabled an amendment calling for a postponement of any reclassification pending a further ACMD review in two years. We urge peers to maintain the trend to evidence-based policy-making by supporting the amendment.

Dr Evan Harris MP Lib Dem science spokesman, David King Former government chief scientific adviser, Professor Michael Rawlins Chair, ACMD 1998-2008, (Lord) Robert May Former government chief scientific adviser, Phil Willis MP Chair, Science select committee, Professor Gabriel Horne Chair, Academy of Medical Science working group on addiction, Professor Colin Blakemore Member, UK Drug Policy Commission; former director, Medical Research Council, Tracey Brown Director, Sense about Science, Dr Leslie King Member, ACMD, Ruth Runciman Former member, ACMD Prof Ian Gilmore, President, Royal College of Physicians and member, Academy of Medical Sciences Working Group on Brain Science, Addiction and Drugs, Prof. Roger Brownsword, Professor of Law at King's College London and member, Academy of Medical Sciences Working Group on Brain Science, Addiction and Drugs, Prof Bill Deakin, Professor of Psychiatry, Manchester University and member, Academy of Medical Sciences Working Group on Brain Science, Addiction and Drugs, Prof Trevor Robbins, Professor of Cognitive Neuroscience, University of Cambridge, member, Academy of Medical Sciences Working Group on Brain Science, Addiction and Drugs